Semiconductor device with metal plug having rounded top surface and method for forming the same

ABSTRACT

A semiconductor device includes a first metal plug and an etch stop layer disposed over a semiconductor substrate. The first metal plug has an upper portion protruding from a top surface of the etch stop layer, and a top surface of the upper portion is rounded. The semiconductor device also includes a second metal plug disposed over the first metal plug. The second metal plug is in direct contact with a first sidewall of the upper portion of the first metal plug and the top surface of the etch stop layer.

TECHNICAL FIELD

The present disclosure relates to a semiconductor device and a methodfor forming the same, and more particularly, to a semiconductor devicewith a metal plug having a rounded top surface and a method for formingthe same.

DISCUSSION OF THE BACKGROUND

Semiconductor devices are essential for many modern applications. Withthe advancement of electronic technology, semiconductor devices arebecoming smaller in size while providing greater functionality andcomprising greater amounts of integrated circuitry. Due to theminiaturized scale of semiconductor devices, various types anddimensions of semiconductor devices performing different functions areintegrated and packaged into a single module. Furthermore, numerousmanufacturing operations are implemented for integration of varioustypes of semiconductor devices.

However, the manufacturing and integration of semiconductor devicesinvolve many complicated steps and operations. An increase in complexityof manufacturing and integration of the semiconductor device may causedeficiencies such as misalignment in interconnect structures.Accordingly, there is a continuous need to improve the structure and themanufacturing process of semiconductor devices.

This Discussion of the Background section is provided for backgroundinformation only. The statements in this Discussion of the Backgroundare not an admission that the subject matter disclosed in this sectionconstitutes prior art to the present disclosure, and no part of thisDiscussion of the Background section may be used as an admission thatany part of this application, including this Discussion of theBackground section, constitutes prior art to the present disclosure.

SUMMARY

In one embodiment of the present disclosure, a semiconductor device isprovided. The semiconductor device includes a first metal plug and anetch stop layer disposed over a semiconductor substrate. The first metalplug has an upper portion protruding from a top surface of the etch stoplayer, and a top surface of the upper portion is rounded. Thesemiconductor device also includes a second metal plug disposed over thefirst metal plug. The second metal plug is in direct contact with afirst sidewall of the upper portion of the first metal plug and the topsurface of the etch stop layer.

In some embodiments, the semiconductor device further includes a firstdielectric layer disposed between the semiconductor substrate and theetch stop layer. The first dielectric layer and the etch stop layersurround a lower portion of the first metal plug. In some embodiments,the semiconductor device further includes a second dielectric layerdisposed over the etch stop layer. The upper portion of the first metalplug has a second sidewall opposite to the first sidewall, and thesecond dielectric layer is in direct contact with the second sidewall.In some embodiments, the second dielectric layer is separated from thefirst sidewall of the upper portion of the first metal plug. In someembodiments, a height of the first sidewall is substantially the same asa height of the second dielectric layer. In some embodiments, thesemiconductor device further includes a third dielectric layer disposedover the second dielectric layer. The third dielectric layer partiallycovers the upper portion of the first metal plug. In some embodiments,the semiconductor device further includes a silicide layer disposedbetween the first metal plug and the second metal plug. The top surfaceof the upper portion of the first metal plug is separated from thesecond metal plug by the silicide layer.

In another embodiment of the present disclosure, a semiconductor deviceis provided. The semiconductor device includes a first metal plug and afirst dielectric layer disposed over a semiconductor substrate. Thesemiconductor device also includes an etch stop layer disposed over thefirst dielectric layer. The first metal plug has an upper portionprotruding from the etch stop layer, the upper portion has a convex topsurface, and the etch stop layer adjoins a lower portion of the firstmetal plug. The semiconductor device further includes a seconddielectric layer disposed over the etch stop layer, and a topmost pointof the convex top surface is higher than a top surface of the seconddielectric layer. In addition, the semiconductor device includes asecond metal plug disposed over the first metal plug. The second metalplug extends to contact a top surface of the etch stop layer.

In some embodiments, the convex top surface is between a first sidewalland a second sidewall of the upper portion of the first metal plug, andthe first sidewall is in direct contact with the second metal plug. Insome embodiments, the second sidewall of the upper portion of the firstmetal plug is in direct contact with the second dielectric layer. Insome embodiments, the semiconductor device further includes a thirddielectric layer disposed over the second dielectric layer, and asilicide layer disposed between the upper portion of the first metalplug and the second metal plug. The silicide layer extends between theconvex top surface of the upper portion and the third dielectric layer.In some embodiments, the second metal plug and the second dielectriclayer are separated by an air gap. In some embodiments, thesemiconductor device further includes a metal-insulator-metal capacitordisposed over the second metal plug. The metal-insulator-metal capacitoris electrically connected to the first metal plug through the secondmetal plug. In some embodiments, the semiconductor device furtherincludes a bit line disposed over the second metal plug. The bit line iselectrically connected to the first metal plug through the second metalplug.

In one embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstdielectric layer over a semiconductor substrate, and forming an etchstop layer over the first dielectric layer. The method also includesforming a second dielectric layer over the etch stop layer, and forminga first metal plug penetrating through the second dielectric layer, theetch stop layer and the first dielectric layer. The first metal plugprotrudes from the second dielectric layer. The method further includesperforming an anisotropic etching process to partially remove the firstmetal plug such that the first metal plug has a convex top surface, andforming a third dielectric layer covering the second dielectric layerand the convex top surface of the first metal plug. In addition, themethod includes forming a second metal plug over the first metal plug.The second metal plug penetrates through the third dielectric layer andextends to contact the etch stop layer.

In some embodiments, after the anisotropic etching process is performed,an edge of the convex top surface of the first metal plug is in directcontact with a top surface of the second dielectric layer. In someembodiments, the method further includes performing a silicidationprocess to form a silicide layer over the convex top surface of thefirst metal plug before the third dielectric layer is formed. In someembodiments, a portion of the silicide layer is sandwiched between thethird dielectric layer and the first metal plug after the second metalplug is formed. In some embodiments, the step of forming the secondmetal plug further includes forming an opening in the third dielectriclayer and forming a gap in the second dielectric layer to partiallyexpose the etch stop layer, and forming the second metal plug in theopening and the gap. The first metal plug has a first sidewall and asecond sidewall opposite to the first sidewall, the first sidewall isexposed by the gap, the second sidewall is covered by the seconddielectric layer, and the second metal plug is in direct contact withthe first sidewall. In some embodiments, the method further includesforming an energy-removable layer between the second metal plug and thethird dielectric layer, forming a conductive layer over the second metalplug, and performing a heat treatment process to transform theenergy-removable layer into an air gap. The energy-removable layer iscovered by the conductive layer.

Embodiments of a semiconductor device and methods for forming the sameare provided. The semiconductor device may include a first metal plugand a second metal plug over the first metal plug. Since the first metalplug has a rounded top surface, the contact area between the first metalplug and the second metal plug is increased, and the electric fieldstrength on the rounded top surface of the first metal plug is evenlydistributed. As a result, the performance and reliability of the devicemay be improved. Moreover, the issues caused by misalignment between thefirst metal plug and the second metal plug may be prevented or reduced.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and advantages of the disclosure will be describedhereinafter, and form the subject of the claims of the disclosure. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present disclosure. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the disclosure as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It shouldbe noted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 2 is a partial enlarged view of region A of FIG. 1, in accordancewith some embodiments.

FIG. 3 is a cross-sectional view illustrating a semiconductor device, inaccordance with some embodiments.

FIG. 4 is a flow diagram illustrating a method of forming asemiconductor device, in accordance with some embodiments.

FIG. 5 is a cross-sectional view illustrating an intermediate stage forforming a photoresist pattern in the formation of a semiconductordevice, in accordance with some embodiments.

FIG. 6 is a cross-sectional view illustrating an intermediate stage forforming openings in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 7 is a cross-sectional view illustrating an intermediate stage forforming a metal layer in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 8 is a cross-sectional view illustrating an intermediate stage forforming metal portions in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 9 is a cross-sectional view illustrating an intermediate stage forforming metal plugs in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 10 is a cross-sectional view illustrating an intermediate stage forforming metal plugs with rounded top surfaces in the formation of asemiconductor device, in accordance with some embodiments.

FIG. 11 is a cross-sectional view illustrating an intermediate stage forforming silicide layers in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 12 is a cross-sectional view illustrating an intermediate stage forforming a dielectric layer in the formation of a semiconductor device,in accordance with some embodiments.

FIG. 13 is a cross-sectional view illustrating an intermediate stage forforming a photoresist pattern in the formation of a semiconductordevice, in accordance with some embodiments.

FIG. 14 is a cross-sectional view illustrating an intermediate stage forforming openings and gaps in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 15 is a cross-sectional view illustrating an intermediate stage forforming an energy-removable material in the formation of a semiconductordevice, in accordance with some embodiments.

FIG. 16 is a cross-sectional view illustrating an intermediate stage forforming a metal layer in the formation of a semiconductor device, inaccordance with some embodiments.

FIG. 17 is a cross-sectional view illustrating an intermediate stage forforming energy-removable layers and metal plugs in the formation of asemiconductor device, in accordance with some embodiments.

FIG. 18 is a cross-sectional view illustrating an intermediate stage forforming a dielectric layer in the formation of a semiconductor device,in accordance with some embodiments.

FIG. 19 is a cross-sectional view illustrating an intermediate stage forforming metal-insulator-metal capacitors in the formation of asemiconductor device, in accordance with some embodiments.

FIG. 20 is a cross-sectional view illustrating an intermediate stage forforming bit lines in the formation of a semiconductor device, inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be disposed between the first and secondfeatures, such that the first and second features may not be in directcontact. In addition, the present disclosure may repeat referencenumerals and/or letters in the various examples. This repetition is forthe purpose of simplicity and clarity and does not in itself dictate arelationship between the various embodiments and/or configurationsdiscussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

FIG. 1 is a cross-sectional view illustrating a semiconductor device 100a, and FIG. 2 is a partial enlarged view of the region A in thesemiconductor device 100 a, in accordance with some embodiments. Asshown in FIGS. 1 and 2, the semiconductor device 100 a includes a firstdielectric layer 103′ and a plurality of first metal plugs 119 a′, 119b′, 119 c′ disposed over a semiconductor substrate 101, in accordancewith some embodiments. Moreover, in some embodiments, an etch stop layer105′ is disposed over the first dielectric layer 103′, and the firstmetal plugs 119 a′, 119 b′ and 119 c′ protrude from the etch stop layer103′.

It should be noted that, although only three first metal plugs areillustrated in FIG. 1, the present disclosure is not limited thereto.Depending on the product requirements, the number of first metal plugsin the semiconductor device structure 100 a may be less or more thanthree. Referring to FIG. 2, the first metal plug 119 a′ has an upperportion P1 protruding from the top surface 105′T of the etch stop layer105′, and a lower portion P2 under the upper portion P1.

In some embodiments, the etch stop layer 105′ and the first dielectriclayer 103′ surround the lower portion P2 of the first metal plug 119 a′,and the upper portion P1 of the first metal plug 119 a′ has a rounded(or curved) top surface TS. In some embodiments, the etch stop layer105′ and the first dielectric layer 103′ adjoin the sidewalls of thelower portion P2. In some embodiments, the top surface TS of the upperportion P1 is convex, and the top surface TS is between a first sidewallSW1 and a second sidewall SW2 of the upper portion P1.

Moreover, the semiconductor device 100 a includes a plurality of secondmetal plugs 137 a, 137 b, 137 c disposed over the first metal plugs 119a′, 119 b′, 119 c′, a second dielectric layer 107″ disposed over theetch stop layer 105′, and a third dielectric layer 123′ disposed overthe second dielectric layer 107″, as shown in FIG. 1 in accordance withsome embodiments. In the present embodiment, the semiconductor device100 a includes a plurality of silicide layers 121 a, 121 b, 121 cdisposed between the first metal plugs 119 a′, 119 b′, 119 c′ and thesecond metal plugs 137 a, 137 b, 137 c. By forming the silicide layers121 a, 121 b, 121 c, the contact resistance between the first metalplugs 119 a′, 119 b′, 119 c′ and the second metal plugs 137 a, 137 b,137 c may be decreased, thereby improving the performance of thesemiconductor device 100 a. However, in some other embodiments, thesilicide layers 121 a, 121 b, 121 c can be omitted.

Referring to FIG. 2, the second metal plug 137 a extends to contact thetop surface 105′T of the etch stop layer 105′, in accordance with someembodiments. In some embodiments, the first sidewall SW1 of the upperportion P1 of the first metal plug 119 a′ is in direct contact with thesecond metal plug 137 a, and the second sidewall SW2 of the upperportion P1 of the first metal plug 119 a′ is in direct contact with thesecond dielectric layer 107″.

It should be noted that the second dielectric layer 107″ is separatedfrom the first sidewall SW1, and a height H1 of the first sidewall SW1is substantially the same as a height H2 of the second dielectric layer107″, in accordance with some embodiments. Within the context of thisdisclosure, the word “substantially” means preferably at least 90%, morepreferably 95%, even more preferably 98%, and most preferably 99%.

More specifically, the top surface TS of the upper portion P1 of thefirst metal plug 119 a′ has a topmost point TP, and the topmost point TPis higher than the top surface 107″T of the second dielectric layer107″. That is, the upper portion P1 of the first metal plug 119 a′protrudes from the top surface 107″T of the second dielectric layer107″, in accordance with some embodiments.

In addition, the top surface TS of the upper portion P1 of the firstmetal plug 119 a′ is separated from the second metal plug 137 a by thesilicide layer 121 a, and the silicide layer 121 a extends between thetop surface TS and the third dielectric layer 123′, in accordance withsome embodiments. In other words, the third dielectric layer 123′partially covers the upper portion P1 of the first metal plug 119 a′, inaccordance with some embodiments.

The second metal plugs 137 a, 137 b, 137 c are separated from the seconddielectric layer 123′ by a plurality of air gaps 136 a 1, 136 a 2, 136 b1, 136 b 2, 136 c 1, 136 c 2, as shown in FIGS. 1 and 2 in accordancewith some embodiments. By forming the air gaps 136 a 1, 136 a 2, 136 b1, 136 b 2, 136 c 1, 136 c 2, the parasitic capacitance between adjacentsecond metal plugs may be reduced, thereby improving the operation speedof the semiconductor device 100 a. However, in some embodiments, the airgaps 136 a 1, 136 a 2, 136 b 1, 136 b 2, 136 c 1, 136 c 2 can beomitted.

It should be noted that the aforementioned features of the region A inthe semiconductor device 100 a are similar to, or the same as, featuresof other regions in the semiconductor device 100 a, such as the regionincluding the first metal plug 119 b′, or the region including the firstmetal plug 119 c′.

Referring to FIG. 1, the semiconductor device 100 a further includes aplurality of metal-insulator-metal (MIM) capacitors 150 a, 150 b, 150 cdisposed over the second metal plugs 137 a, 137 b, 137 c, a fourthdielectric layer 139 surrounding the MIM capacitors 150 a, 150 b, 150 c,and a filling dielectric layer 161 filling the space within the MIMcapacitors 150 a, 150 b, 150 c, in accordance with some embodiments. Itshould be noted that the MIM capacitors 150 a, 150 b, 150 c areelectrically connected to the first metal plugs 119 a′, 119 b′, 119 c′through the second metal plugs 137 a, 137 b, 137 c, in accordance withsome embodiments.

More specifically, in some embodiments, the MIM capacitor 150 a includesa metal layer 151 a, an insulating layer 153 a and a metal layer 155 a.Similarly, the MIM capacitor 150 b includes a metal layer 151 b, aninsulating layer 153 b and a metal layer 155 b, and the MIM capacitor150 c includes a metal layer 151 c, an insulating layer 153 c and ametal layer 155 c.

In some embodiments, the metal layer 151 a is U-shaped, and the secondmetal plug 137 a is covered by the metal layer 151 a. Moreover, themetal layer 151 a is surrounded by the insulating layer 153 a, and theinsulating layer 153 a is surrounded by the metal layer 155 a, as shownin FIG. 1 in accordance with some embodiments. The details of the metaland insulating layers in the MIM capacitors 150 b and 150 c are similarto, or the same as, those of the MIM capacitor 150 a, and descriptionsthereof are not repeated herein.

In some embodiments, the filling dielectric layer 161 fills the spacewithin each of the MIM capacitors 150 a, 150 b, 150 c, and the MIMcapacitors 150 a, 150 b, 150 c are separated from each other by thefourth dielectric layer 139. In some embodiments, the semiconductordevice 100 a is a dynamic random access memory (DRAM).

In the semiconductor device 100 a, since the first metal plugs 119 a′,119 b′, 119 c′ have rounded top surfaces, the contact areas between thefirst metal plugs 119 a′, 119 b′, 119 c′ and the second metal plugs 137a, 137 b, 137 c (or the contact areas between the silicide layers 121 a,121 b, 121 c and the second metal plugs 137 a, 137 b, 137 c) areincreased, compared with arrangements in which the first metal plugs 119a′, 119 b′, 119 c′ have flat top surfaces and the second metal plugs 137a, 137 b, 137 c are perfectly aligned with the first metal plugs 119 a′,119 b′, 119 c′. The rounded top surfaces of the first metal plugs 119a′, 119 b′, 119 c′ may cause a corresponding decrease of the resistancesbetween the first metal plugs 119 a′, 119 b′, 119 c′ and the secondmetal plugs 137 a, 137 b, 137 c, thereby improving the overall deviceperformance.

Moreover, the electric field strengths on the rounded top surfaces ofthe first metal plugs 119 a′, 119 b′, 119 c′ are evenly distributed,since the first metal plugs 119 a′, 119 b′, 119 c′ have no sharpportions. Therefore, the lifespan of the semiconductor device 100 a maybe extended significantly, and the performance and reliability of thedevice may be improved. Furthermore, since the etch stop layer 105′adjoins sidewalls of the first metal plugs 119 a′, 119 b′, 119 c′, theunderlying electronic components can be prevented from being exposedduring the processes for forming the second metal plugs 137 a, 137 b,137 c, and the issues caused by misalignment between the first metalplugs 119 a′, 119 b′, 119 c′ and the second metal plugs 137 a, 137 b,137 c may be prevented or reduced.

FIG. 3 is a cross-sectional view illustrating a semiconductor device 100b, which is an alternative embodiment of the semiconductor device 100 a,in accordance with some embodiments. For reasons of consistency andclarity, similar components appearing in both FIGS. 1 and 3 will belabeled the same.

One difference between the embodiment shown in FIG. 3 and the embodimentshown in FIG. 1 is that the MIM capacitors 150 a, 150 b, 150 c arereplaced by a plurality of bit lines 250 a, 250 b, 250 c in theembodiment shown in FIG. 3. It should be noted that the bit lines 250 a,250 b, 250 c are electrically connected to the first metal plugs 119 a′,119 b′, 119 c′ through the second metal plugs 137 a, 137 b, 137 c, inaccordance with some embodiments.

FIG. 4 is a flow diagram illustrating a method 10 of forming thesemiconductor device 100 a or 100 b, and the method 10 includes stepsS11, S13, S15, S17 and S19, in accordance with some embodiments. Thesteps S11 to S19 of FIG. 4 are elaborated in connection with otherfigures of the present disclosure, which are cross-sectional viewsillustrating sequential intermediate stages in the formation of thesemiconductor device 100 a or 100 b, in accordance with someembodiments.

As shown in FIG. 5, the semiconductor substrate 101 is provided. Thesemiconductor substrate 101 may be a portion of an integrated circuit(IC) chip that includes various passive and active microelectronicdevices, such as resistors, capacitors, inductors, diodes, p-typefield-effect transistors (pFETs), n-type field-effect transistors(nFETs), metal-oxide semiconductor field-effect transistors (MOSFETs),complementary metal-oxide semiconductor (CMOS) transistors, bipolarjunction transistors (BJTs), laterally diffused MOS (LDMOS) transistors,high voltage transistors, high frequency transistors, fin field-effecttransistors (FinFETs), other suitable IC components, or combinationsthereof.

Depending on the IC fabrication stage, the semiconductor substrate 101may include various material layers (e.g., dielectric layers,semiconductor layers, and/or conductive layers) configured to form ICfeatures (e.g., doped regions, isolation features, gate features,source/drain features, interconnect features, other features, orcombinations thereof). The semiconductor substrate 101 has beensimplified for the sake of clarity. It should be noted that additionalfeatures can be added in the semiconductor substrate 101, and some ofthe features described below can be replaced, modified, or eliminated inother embodiments.

In some embodiments, the first dielectric layer 103, the etch stop layer105, and the second dielectric layer 107 are sequentially disposed overthe semiconductor substrate 101. The respective step is illustrated asthe step S11 in the method 10 shown in FIG. 4.

In some embodiments, the first dielectric layer 103 is made of siliconoxide, silicon carbide, silicon nitride, silicon oxynitride, anothersuitable material, or a combination thereof, and the first dielectriclayer 103 is formed by a deposition process, such as a chemical vapordeposition (CVD) process, a physical vapor deposition (PVD) process, anatomic layer deposition (ALD) process, a spin-on process, or anothersuitable process.

Some materials and processes used to form the etch stop layer 105 andthe second dielectric layer 107 are similar to, or the same as, thoseused to form the first dielectric layer 103, and descriptions thereofare not repeated herein. It should be noted that the material of theetch stop layer 105 is different from the material of the seconddielectric layer 107, in accordance with some embodiments.

After the second dielectric layer 107 is formed, a photoresist pattern109 with openings 112 is disposed over the second dielectric layer 107,and the second dielectric layer 107 is exposed by the openings 112, asshown in FIG. 5 in accordance with some embodiments. In someembodiments, the photoresist pattern 109 may be formed by a depositionprocess and a patterning process.

The deposition process for forming the photoresist pattern 109 mayinclude a CVD process, a high-density plasma chemical vapor deposition(HDPCVD) process, a spin-on process, or another suitable process. Thepatterning process for forming the photoresist pattern 109 may include aphotolithography process. The photolithography process may includephotoresist coating (e.g., spin coating), soft baking, mask aligning,exposure, post-exposure baking, developing the photoresist, rinsing anddrying (e.g., hard baking).

Next, an etching process is performed on the structure by using thephotoresist pattern 109 as a mask, as shown in FIG. 6 in accordance withsome embodiments. It should be noted that the etching process isperformed until the top surface 101T of the semiconductor substrate 101is exposed, and openings 114 are formed under the openings 112.

After the etching process, the openings 114 are surrounded by theremaining second dielectric layer 107′, the remaining etch stop layer105′ and the remaining first dielectric layer 103′. The etching processmay be a dry etching process, a wet etching process, or a combinationthereof.

After the openings 114 are formed, a metal layer 117 is deposited tofill the openings 114, as shown in FIG. 7 in accordance with someembodiments. More specifically, the openings 114 are entirely filled bythe metal layer 117, and the metal layer 117 extends onto thephotoresist pattern 109, in accordance with some embodiments.

In some embodiments, the metal layer 117 is made of copper (Cu). In someother embodiments, the metal layer 117 is made of tungsten (W), cobalt(Co), titanium (Ti), aluminum (Al), tantalum (Ta), or another applicablematerial. Moreover, in some embodiments, the metal layer 117 is formedby a CVD process, a PVD process, an ALD process, a plating (e.g.,electroplating) process, a sputtering process, or another suitableprocess.

Next, excess portions of the metal layer 117 are removed to form metalportions 117 a, 117 b, 117 c, as shown in FIG. 8 in accordance with someembodiments. More specifically, the portions of the metal layer 117covering the photoresist pattern 109 are removed, and the portions ofthe metal layer 117 deposited into the openings 112 and 114 remain, inaccordance with some embodiments.

In some embodiments, the removal of the excess portions of the metallayer 117 is performed by a planarization process or an etching process.The planarization process may be a chemical mechanical polishing (CMP)process.

After the metal portions 117 a, 117 b, 117 c are formed, the photoresistpattern 109 is removed, as shown in FIG. 9 in accordance with someembodiments. In some embodiments, the portions of the metal portions 117a, 117 b, 117 c protruding from a top surface 107′T of the seconddielectric layer 107′ are slightly etched during the removal process ofthe photoresist pattern 109, and first metal plugs 119 a, 119 b, 119 care obtained (i.e., the remaining metal portions 117 a, 117 b, 117 c).

In particular, each of the first metal plugs 119 a, 119 b, 119 cpenetrates through the second dielectric layer 107′, the etch stop layer105′ and the first dielectric layer 103′ to electrically connect to theelectronic components in the semiconductor substrate 101, in accordancewith some embodiments. The respective step is illustrated as the stepS13 in the method 10 shown in FIG. 4. It should be noted that the firstmetal plugs 119 a, 119 b, 119 c still protrude from the top surface107′T of the second dielectric layer 107′, in accordance with someembodiments.

An anisotropic etching process is performed to partially remove thefirst metal plugs 119 a, 119 b, 119 c such that each of the etched firstmetal plugs 119 a′, 119 b′, 119 c′ has a rounded (or curved) top surfaceTS, as illustrated in FIG. 10. In some embodiments, the top surfaces TSof the first metal plugs 119 a′, 119 b′, 119 c′ are convex surfaces. Therespective step is illustrated as the step S15 in the method 10 shown inFIG. 4.

As described previously, the topmost points TP of the top surfaces TS ofthe first metal plugs 119 a′, 119 b′, 119 c′ are higher than the topsurface 107′T of the second dielectric layer 107′. More specifically,the edges E of the top surfaces TS are in direct contact with the topsurface 107′T of the second dielectric layer 107′, in accordance withsome embodiments. For the purpose of simplicity and clarity, the topsurfaces TS, the topmost points TP and the edges E are indicated only inthe first metal plug 119 a′ of FIG. 10. However, the first metal plugs119 b′ and 119 c′ of FIG. 10 may have features similar to those of thefirst metal plug 119 a′. In some embodiments, the anisotropic etchingprocess is a dry etching process.

Next, silicide layers 121 a, 121 b, 121 c are disposed over the firstmetal plugs 119 a′, 119 b′ and 119 c′ by a silicidation process, asshown in FIG. 11 in accordance with some embodiments. In someembodiments, the silicidation process includes a metal materialdeposition process and an annealing process performed in sequence. Insome embodiments, the deposition process of the silicidation processincludes a PVD process, an ALD process, or another suitable process.After the annealing process, the unreacted metal material is removed.

In some embodiments, the silicide layers 121 a, 121 b, 121 c are made ofone of more of copper silicide, tungsten silicide, cobalt silicide,titanium silicide, nickel silicide, and molybdenum silicide. Asdescribed previously, by forming the silicide layers 121 a, 121 b, 121c, the contact resistance between the first metal plugs 119 a′, 119 b′,119 c′ and the overlying conductive components (e.g., the second metalplugs 137 a, 137 b, 137 c as shown in FIGS. 1 and 3) may be decreased,thereby improving the performance of the device. In some otherembodiments, the silicidation process is not performed and the silicidelayers 121 a, 121 b, 121 c are not formed.

The third dielectric layer 123 is formed to cover the second dielectriclayer 107′ and the first metal plugs 119 a′, 119 b′, 119 c′, as shown inFIG. 12 in accordance with some embodiments. The respective step isillustrated as the step S17 in the method 10 shown in FIG. 4. In someembodiments, the silicide layers 121 a, 121 b, 121 c are covered by thethird dielectric layer 123.

It should be noted that the top surfaces TS of the first metal plugs 119a′, 119 b′, 119 c′ are separated from the third dielectric layer 123 bythe silicide layers 121 a, 121 b, 121 c, in accordance with someembodiments. Some materials and processes used to form the thirddielectric layer 123 are similar to, or the same as, those used to formthe first dielectric layer 103, and descriptions thereof are notrepeated herein. It should also be noted that the material of the thirddielectric layer 123 is different from the material of the etch stoplayer 105′, in accordance with some embodiments.

After the third dielectric layer 123 is formed, a photoresist pattern125 with openings 128 is disposed over the third dielectric layer 123,and the third dielectric layer 123 is exposed by the openings 128, asshown in FIG. 13 in accordance with some embodiments. Some materials andprocesses used to form the photoresist pattern 125 are similar to, orthe same as, those used to form the photoresist pattern 109 of FIG. 5,and descriptions thereof are not repeated herein.

Next, an etching process is performed on the structure by using thephotoresist pattern 125 as a mask, as shown in FIG. 14 in accordancewith some embodiments. It should be noted that the etching process isperformed until the silicide layers 121 a, 121 b, 121 c or the firstmetal plugs 119 a′, 119 b′, 119 c′ are exposed, and openings 130 areformed in the remaining third dielectric layer 123′ and gaps 132 areformed in the remaining second dielectric layer 107″, in accordance withsome embodiments. In addition, the etching process may be a dry etchingprocess, a wet etching process, or a combination thereof.

In some embodiments, the positions of the openings 128 over the seconddielectric layer 107′ (see FIG. 13), causes the openings 130 to exposeportions of the second dielectric layer 107′ during the etching process,and the exposed portions of the second dielectric layer 107′ are removedto form the gaps 132. It should be noted that the top surface 105′T ofthe etch stop layer 105′ is exposed by the gaps 132, in accordance withsome embodiments. As mentioned above, the etch stop layer 105′ canprotect the underlying electronic components from being exposed duringthe etching process.

Moreover, each of the first metal plugs 119 a′, 119 b′, 119 c′ has afirst sidewall SW1 and a second sidewall SW2 opposite to the firstsidewall SW1. In some embodiments, the first sidewalls SW1 are exposedby the gaps 132, and the second sidewalls SW2 remain covered by thesecond dielectric layer 107″. In some embodiments, portions of thesilicide layers 121 a, 121 b, 121 c are sandwiched between the thirddielectric layer 123′ and the first metal plugs 119 a′, 119 b′, 119 c′after the openings 130 and the gaps 132 are formed.

An energy-removable material 135 is disposed over sidewalls of theopenings 128, 130 and the gaps 132 of FIG. 14, such that reducedopenings 128′, 130′ and reduced gaps 132′ are obtained, as shown in FIG.15 in accordance with some embodiments.

In some embodiments, the energy-removable material 135 includes athermal-decomposable material. In some other embodiments, theenergy-removable material 135 includes a photonic-decomposable material,an e-beam decomposable material, or another suitable energy-decomposablematerial. Specifically, in some embodiments, the energy-removablematerial 135 includes a base material and a decomposable porogenmaterial that is substantially removed upon exposure to an energy source(e.g., heat).

In some embodiments, the base material includes hydrogen silsesquioxane(HSQ), methylsilsesquioxane (MSQ), porous polyarylether (PAE), porousSiLK, or porous silicon oxide (SiO₂), and the decomposable porogenmaterial includes a porogen organic compound, which can, in thesubsequent processes, provide porosity to the space previously occupiedby the energy-removable material 135.

In some embodiments, the energy-removable material 135 is formed by adeposition process and an etching process. In some embodiments, thedeposition process includes CVD, PVD, ALD, spin coating, or anothersuitable process, and the etching process includes a reactive ionetching (RIE) process, which is used to remove the excess portions overthe photoresist pattern 125.

Next, the openings 128′ and 130′ and the gaps 132′ are filled by a metallayer 137, and the metal layer 137 extends onto the photoresist pattern125, as shown in FIG. 16 in accordance with some embodiments. It shouldbe noted that the metal layer 137 is in direct contact with the firstmetal plugs 119 a′, 119 b′, 119 c′ and the etch stop layer 105′.

In some embodiments, the metal layer 137 is made of copper (Cu). In someother embodiments, the metal layer 137 is made of tungsten (W), cobalt(Co), titanium (Ti), aluminum (Al), tantalum (Ta), or another applicablematerial. Moreover, in some embodiments, the metal layer 137 is formedby a CVD process, a PVD process, an ALD process, a plating (e.g.,electroplating) process, a sputtering process, or another suitableprocess.

Next, a planarization process is performed to remove the photoresistpattern 125 and the excess portions of the metal layer 137 and theenergy-removable material 135 above the third dielectric layer 123′, asshown in FIG. 17 in accordance with some embodiments. The planarizationprocess may be a CMP process. After the planarization process, thesecond metal plugs 137 a, 137 b, 137 c, and the energy-removable layers135′ are obtained. The respective step is illustrated as the step S19 inthe method 10 shown in FIG. 4.

It should be noted that the second metal plugs 137 a, 137 b, 137 c aredisposed over and are electrically connected to the first metal plugs119 a′, 119 b′, 119 c′, respectively, and the energy-removable layers135′ are located between the second metal plugs 137 a, 137 b, 137 c andthe third dielectric layer 123′, in accordance with some embodiments.

After the second metal plugs 137 a, 137 b, 137 c are formed, a fourthdielectric layer 139 is formed covering the third dielectric layer 123′,the energy-removable layers 135′ and the second metal plugs 137 a, 137b, 137 c, as shown in FIG. 18 in accordance with some embodiments. Somematerials and processes used to form the fourth dielectric layer 139 aresimilar to, or the same as, those used to form the first dielectriclayer 103, and descriptions thereof are not repeated herein.

The MIM capacitors 150 a, 150 b, 150 c are formed in the fourthdielectric layer 139, as shown in FIG. 19 in accordance with someembodiments. In some embodiments, the metal layers 151 a, 151 b, 151 care U-shaped, and the energy-removable layers 135′ and the second metalplugs 137 a, 137 b, 137 c are covered by the corresponding metal layers151 a, 151 b, 151 c. Moreover, in some embodiments, the metal layers 151a, 151 b, 151 c are surrounded by the insulating layers 153 a, 153 b,153 c, respectively, and the insulating layers 153 a, 153 b, 153 c aresurrounded by the metal layers 155 a, 155 b, 155 c, respectively.Furthermore, the remaining space within each of the MIM capacitors 150a, 150 b, 150 c may be filled by the filling dielectric layer 161.

Some materials and processes used to form the metal layers 151 a, 151 b,151 c and the metal layers 155 a, 155 b, 155 c are similar to, or thesame as, those used to form the second metal plugs 137 a, 137 b, 137 c,and descriptions thereof are not repeated herein. Some materials andprocesses used to form the insulating layers 153 a, 153 b, 153 c and thefilling dielectric layer 161 are similar to, or the same as, those usedto form the first dielectric layer 103, and descriptions thereof are notrepeated herein. In some embodiments, the filling dielectric layer 161and the fourth dielectric layer 139 are made of the same material, andno obvious interface exists between the filling dielectric layer 161 andthe fourth dielectric layer 139.

After the MIM capacitors 150 a, 150 b, 150 c are formed, a heattreatment is used to remove the decomposable porogen material of theenergy-removable layers 135′ to generate pores, and the pores are filledby air such that the air gaps 136 a 1, 136 a 2, 136 b 1, 136 b 2, 136 c1, 136 c 2 are obtained between the second metal plugs 137 a, 137 b, 137c and the third dielectric layer 123′, as shown in FIG. 1 in accordancewith some embodiments.

In some other embodiments, the heat treatment process can be replaced bya light treatment process, an e-beam treatment process, a combinationthereof, or another applicable energy treatment process. For example, anultraviolet (UV) light or laser light may be used to remove thedecomposable porogen material of the energy-removable layers 135′, suchthat the air gaps 136 a 1, 136 a 2, 136 b 1, 136 b 2, 136 c 1, 136 c 2are obtained.

As described previously, since the dielectric constant of air isrelatively low, the parasitic capacitance between the second metal plugs137 a, 137 b, 137 c may be reduced, thereby improving the operationspeed of the semiconductor device 100 a. In some other embodiments, theenergy-removable layers 135′ and the air gaps 136 a 1, 136 a 2, 136 b 1,136 b 2, 136 c 1, 136 c 2 are not formed. In these cases, the secondmetal plugs 137 a, 137 b, 137 c may be in direct contact with the thirddielectric layer 123′.

In an alternative embodiment, the bit lines 250 a, 250 b, 250 c areformed in the fourth dielectric layer 139, as shown in FIG. 20 inaccordance with some embodiments. In some embodiments, theenergy-removable layers 135′ and the second metal plugs 137 a, 137 b,137 c are covered by the corresponding bit lines 250 a, 250 b, 250 c.Some materials and processes used to form the bit lines 250 a, 250 b,250 c are similar to, or the same as, those used to form the secondmetal plugs 137 a, 137 b, 137 c, and descriptions thereof are notrepeated herein.

After the bit lines 250 a, 250 b, 250 c are formed, the air gaps 136 a1, 136 a 2, 136 b 1, 136 b 2, 136 c 1, 136 c 2 are formed by the processdescribed above, as shown in FIG. 3 in accordance with some embodiments.After the air gaps 136 a 1, 136 a 2, 136 b 1, 136 b 2, 136 c 1, 136 c 2are formed, the semiconductor device 100 b is obtained. In some otherembodiments, the air gaps 136 a 1, 136 a 2, 136 b 1, 136 b 2, 136 c 1,136 c 2 of the semiconductor device 100 b are not formed.

Embodiments of the semiconductor devices 100 a, 100 b and methods forforming the same are provided. The semiconductor devices 100 a and 100 binclude the first metal plugs 119 a′, 119 b′, 119 c′ and the secondmetal plugs 137 a, 137 b, 137 c over the first metal plugs 119 a′, 119b′, 119 c′. Because the first metal plugs 119 a′, 119 b′, 119 c′ haverounded (or curved) top surfaces TS, the contact areas between the firstmetal plugs 119 a′, 119 b′, 119 c′ and the second metal plugs 137 a, 137b, 137 c (or the contact areas between the silicide layers 121 a, 121 b,121 c and the second metal plugs 137 a, 137 b, 137 c) are increased,compared with arrangements in which the first metal plugs 119 a′, 119b′, 119 c′ have flat top surfaces and the second metal plugs 137 a, 137b, 137 c are perfectly aligned with the first metal plugs 119 a′, 119b′, 119 c′. The increased contact areas of the rounded top surfaces TSmay cause a corresponding decrease of the resistances between the firstmetal plugs 119 a′, 119 b′, 119 c′ and the second metal plugs 137 a, 137b, 137 c, thereby improving the overall device performance.

Moreover, the electric field strengths on the rounded top surfaces TS ofthe first metal plugs 119 a′, 119 b′, 119 c′ are evenly distributed,since the first metal plugs 119 a′, 119 b′, 119 c′ have no sharpportions. Therefore, the lifespan of the semiconductor devices 100 a and100 b may be extended significantly, and the performance and reliabilityof the devices may be improved.

Furthermore, because the etch stop layer 105′ adjoins sidewalls of thefirst metal plugs 119 a′, 119 b′, 119 c′, the underlying electroniccomponents can be prevented from being exposed during the processes forforming the second metal plugs 137 a, 137 b, 137 c, and the issuescaused by misalignment between the first metal plugs 119 a′, 119 b′, 119c′ and the second metal plugs 137 a, 137 b, 137 c may be prevented orreduced.

In one embodiment of the present disclosure, a semiconductor device isprovided. The semiconductor device includes a first metal plug and anetch stop layer disposed over a semiconductor substrate. The first metalplug has an upper portion protruding from a top surface of the etch stoplayer, and a top surface of the upper portion is rounded. Thesemiconductor device also includes a second metal plug disposed over thefirst metal plug. The second metal plug is in direct contact with afirst sidewall of the upper portion of the first metal plug and the topsurface of the etch stop layer.

In some embodiments, the semiconductor device further includes a firstdielectric layer disposed between the semiconductor substrate and theetch stop layer. The first dielectric layer and the etch stop layersurround a lower portion of the first metal plug. In some embodiments,the semiconductor device further includes a second dielectric layerdisposed over the etch stop layer. The upper portion of the first metalplug has a second sidewall opposite to the first sidewall, and thesecond dielectric layer is in direct contact with the second sidewall.In some embodiments, the second dielectric layer is separated from thefirst sidewall of the upper portion of the first metal plug. In someembodiments, a height of the first sidewall is substantially the same asa height of the second dielectric layer. In some embodiments, thesemiconductor device further includes a third dielectric layer disposedover the second dielectric layer. The third dielectric layer partiallycovers the upper portion of the first metal plug. In some embodiments,the semiconductor device further includes a silicide layer disposedbetween the first metal plug and the second metal plug. The top surfaceof the upper portion of the first metal plug is separated from thesecond metal plug by the silicide layer.

In another embodiment of the present disclosure, a semiconductor deviceis provided. The semiconductor device includes a first metal plug and afirst dielectric layer disposed over a semiconductor substrate. Thesemiconductor device also includes an etch stop layer disposed over thefirst dielectric layer. The first metal plug has an upper portionprotruding from the etch stop layer, the upper portion has a convex topsurface, and the etch stop layer adjoins a lower portion of the firstmetal plug. The semiconductor device further includes a seconddielectric layer disposed over the etch stop layer, and a topmost pointof the convex top surface is higher than a top surface of the seconddielectric layer. In addition, the semiconductor device includes asecond metal plug disposed over the first metal plug. The second metalplug extends to contact a top surface of the etch stop layer.

In some embodiments, the convex top surface is between a first sidewalland a second sidewall of the upper portion of the first metal plug, andthe first sidewall is in direct contact with the second metal plug. Insome embodiments, the second sidewall of the upper portion of the firstmetal plug is in direct contact with the second dielectric layer. Insome embodiments, the semiconductor device further includes a thirddielectric layer disposed over the second dielectric layer, and asilicide layer disposed between the upper portion of the first metalplug and the second metal plug. The silicide layer extends between theconvex top surface of the upper portion of the first metal plug and thethird dielectric layer. In some embodiments, the second metal plug andthe second dielectric layer are separated by an air gap. In someembodiments, the semiconductor device further includes ametal-insulator-metal capacitor disposed over the second metal plug. Themetal-insulator-metal capacitor is electrically connected to the firstmetal plug through the second metal plug. In some embodiments, thesemiconductor device further includes a bit line disposed over thesecond metal plug. The bit line is electrically connected to the firstmetal plug through the second metal plug.

In one embodiment of the present disclosure, a method for forming asemiconductor device is provided. The method includes forming a firstdielectric layer over a semiconductor substrate, and forming an etchstop layer over the first dielectric layer. The method also includesforming a second dielectric layer over the etch stop layer, and forminga first metal plug penetrating through the second dielectric layer, theetch stop layer and the first dielectric layer. The first metal plugprotrudes from the second dielectric layer. The method further includesperforming an anisotropic etching process to partially remove the firstmetal plug such that the first metal plug has a convex top surface, andforming a third dielectric layer covering the second dielectric layerand the convex top surface of the first metal plug. In addition, themethod includes forming a second metal plug over the first metal plug.The second metal plug penetrates through the third dielectric layer andextends to contact the etch stop layer.

In some embodiments, after the anisotropic etching process is performed,an edge of the convex top surface of the first metal plug is in directcontact with a top surface of the second dielectric layer. In someembodiments, the method further includes performing, before the thirddielectric layer is formed, a silicidation process to form a silicidelayer over the convex top surface of the first metal plug. In someembodiments, after the second metal plug is formed, a portion of thesilicide layer is sandwiched between the third dielectric layer and thefirst metal plug. In some embodiments, the step of forming the secondmetal plug further includes forming an opening in the third dielectriclayer and forming a gap in the second dielectric layer to partiallyexpose the etch stop layer, and forming the second metal plug in theopening and in the gap. The first metal plug has a first sidewall and asecond sidewall opposite to the first sidewall, the first sidewall isexposed by the gap, the second sidewall is covered by the seconddielectric layer, and the second metal plug is in direct contact withthe first sidewall. In some embodiments, the method further includesforming an energy-removable layer between the second metal plug and thethird dielectric layer, forming a conductive layer over the second metalplug, and performing a heat treatment process to transform theenergy-removable layer into an air gap. The energy-removable layer iscovered by the conductive layer.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present disclosure, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein, may be utilized according tothe present disclosure. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, and steps.

What is claimed is:
 1. A semiconductor device, comprising: a first metalplug and an etch stop layer disposed over a semiconductor substrate,wherein the first metal plug has an upper portion protruding from a topsurface of the etch stop layer, and wherein a top surface of the upperportion is rounded; and a second metal plug disposed over the firstmetal plug, wherein the second metal plug is in direct contact with afirst sidewall of the upper portion of the first metal plug and the topsurface of the etch stop layer.
 2. The semiconductor device of claim 1,further comprising: a first dielectric layer disposed between thesemiconductor substrate and the etch stop layer, wherein the firstdielectric layer and the etch stop layer surround a lower portion of thefirst metal plug.
 3. The semiconductor device of claim 1, furthercomprising: a second dielectric layer disposed over the etch stop layer,wherein the upper portion of the first metal plug has a second sidewallopposite to the first sidewall, and the second dielectric layer is indirect contact with the second sidewall.
 4. The semiconductor device ofclaim 3, wherein the second dielectric layer is separated from the firstsidewall of the upper portion of the first metal plug.
 5. Thesemiconductor device of claim 3, wherein a height of the first sidewallis substantially the same as a height of the second dielectric layer. 6.The semiconductor device of claim 3, further comprising: a thirddielectric layer disposed over the second dielectric layer, wherein thethird dielectric layer partially covers the upper portion of the firstmetal plug.
 7. The semiconductor device of claim 1, further comprising:a silicide layer disposed between the first metal plug and the secondmetal plug, wherein the top surface of the upper portion of the firstmetal plug is separated from the second metal plug by the silicidelayer.
 8. A semiconductor device, comprising: a first metal plug and afirst dielectric layer disposed over a semiconductor substrate; an etchstop layer disposed over the first dielectric layer, wherein the firstmetal plug has an upper portion protruding from the etch stop layer, theupper portion of the first metal plug has a convex top surface, and theetch stop layer adjoins a lower portion of the first metal plug; asecond dielectric layer disposed over the etch stop layer, wherein atopmost point of the convex top surface is higher than a top surface ofthe second dielectric layer; and a second metal plug disposed over thefirst metal plug, wherein the second metal plug extends to contact a topsurface of the etch stop layer.
 9. The semiconductor device of claim 8,wherein the convex top surface is between a first sidewall and a secondsidewall of the upper portion of the first metal plug, and the firstsidewall is in direct contact with the second metal plug.
 10. Thesemiconductor device of claim 9, wherein the second sidewall of theupper portion of the first metal plug is in direct contact with thesecond dielectric layer.
 11. The semiconductor device of claim 8,further comprising: a third dielectric layer disposed over the seconddielectric layer; and a silicide layer disposed between the upperportion of the first metal plug and the second metal plug, wherein thesilicide layer extends between the convex top surface of the upperportion of the first metal plug and the third dielectric layer.
 12. Thesemiconductor device of claim 8, wherein the second metal plug and thesecond dielectric layer are separated by an air gap.
 13. Thesemiconductor device of claim 8, further comprising: ametal-insulator-metal capacitor disposed over the second metal plug,wherein the metal-insulator-metal capacitor is electrically connected tothe first metal plug through the second metal plug.
 14. Thesemiconductor device of claim 8, further comprising: a bit line disposedover the second metal plug, wherein the bit line is electricallyconnected to the first metal plug through the second metal plug.
 15. Amethod for forming a semiconductor device, comprising: forming a firstdielectric layer over a semiconductor substrate; forming an etch stoplayer over the first dielectric layer; forming a second dielectric layerover the etch stop layer; forming a first metal plug penetrating throughthe second dielectric layer, the etch stop layer and the firstdielectric layer, wherein the first metal plug protrudes from the seconddielectric layer; performing an anisotropic etching process to partiallyremove the first metal plug such that the first metal plug has a convextop surface; forming a third dielectric layer covering the seconddielectric layer and the convex top surface of the first metal plug; andforming a second metal plug over the first metal plug, wherein thesecond metal plug penetrates through the third dielectric layer andextends to contact the etch stop layer.
 16. The method for forming asemiconductor device of claim 15, wherein, after the anisotropic etchingprocess, an edge of the convex top surface of the first metal plug is indirect contact with a top surface of the second dielectric layer. 17.The method for forming a semiconductor device of claim 15, furthercomprising: performing a silicidation process to form a silicide layerover the convex top surface of the first metal plug before the thirddielectric layer is formed.
 18. The method for forming a semiconductordevice of claim 17, wherein a portion of the silicide layer issandwiched between the third dielectric layer and the first metal plugafter the second metal plug is formed.
 19. The method for forming asemiconductor device of claim 15, wherein the step of forming the secondmetal plug further comprises: forming an opening in the third dielectriclayer and a gap in the second dielectric layer to partially expose theetch stop layer, wherein the first metal plug has a first sidewall and asecond sidewall opposite to the first sidewall, the first sidewall isexposed by the gap, and the second sidewall is covered by the seconddielectric layer; and forming the second metal plug in the opening andthe gap such that the second metal plug is in direct contact with thefirst sidewall.
 20. The method for forming a semiconductor device ofclaim 15, further comprising: forming an energy-removable layer betweenthe second metal plug and the third dielectric layer; forming aconductive layer over the second metal plug, wherein theenergy-removable layer is covered by the conductive layer; andperforming a heat treatment process to transform the energy-removablelayer into an air gap.